Are you saying that the couple broke up because the guy is into gambling? He should be happy, because that's a bad partner, if she doesn't like that he is gambling why not start from forbidding him not to gamble anymore first?

She just have to watch over him and see if he will break the promise again, let's say I am the one in this position, I would rather panic about him becoming addicted to gambling rather than trying to stop him entirely from gambling.

Is he a responsible gambler? This is what every couple needs to find out about each other first, they should stop assuming that since one is into gambling it will affect their life together.